The adhesive organs of some arthropods and geckos disclose a promising concept for temporary, reversible adhesion. Their fibrillar nano- or microstructure enables forward movement through uniform adhesive contact on substrates of a great variety of materials also varying greatly in roughness. Since their discovery, these purely physically acting adhesion systems have been the subject of research to generate artificial systems of this nature. Successful concepts for mimicking fibrillar adhesion systems on the basis of elastomeric microstructures have been demonstrated in the very recent past for very smooth and hard substrate surfaces such as glass, silicon wafer or polished metal surfaces. Less successful so far, however, is the transfer of such adhesion mechanisms to rough and yielding surfaces, although these are not a rarity in nature and present no obstacle for arthropods and geckos.
Design parameters such as fibril diameter, aspect ratio and the geometrical shape of the contact point can be specifically varied by means of microstructuring techniques and then studied and analyzed. For example, it was found that fibrils with high aspect ratios better dissipate the elastic energy during detachment and consequently enable the obtention of greater adhesive forces. However, high aspect ratios are disadvantageous for the mechanical stability of such structures, since under compression these are liable to Euler buckling or agglomeration of adjacent fibrils takes place even without the action of force, owing to the decreased bending strength.
Fibrillar structures increase the elastic flexibility in the contact area, as a result of which these can adapt to rough substrates better than unstructured surfaces of the same material. It has been shown that with increasing roughness the adhesive strength decreases, but this effect can be countered with increased flexibility of the fibrillar structures. Theoretically, this can be described through the determination of the effective E modulus, in which the intrinsic E modulus of the solid is decreased as a function of the interpenetrating air volume. Thus, with the example of the β-keratin fibrils of the gecko, a reduction of the E modulus over 4 orders of magnitude was seen, to an effective elasticity modulus of ca. 100 kpA. In addition, hierarchically organized fibril structures have the ability to deal with rugosities over several scales of length, with each hierarchical step covering a defined order of magnitude.
In spite of the enormous increase in fundamental understanding concerning fibrillar adhesion systems in the last decade, their field of application is as yet limited almost exclusively to hard and smooth objects such as glass, wafer and polished metal surfaces. The adhesion mechanisms of such systems on soft, flexible substrates, often with viscoelastic properties, such as for example skin surfaces, are as yet only very little known. Further, in addition to its viscoelastic nature, skin is rough and moist to varying extents and thus represents a major challenge for research.
An important field of reversible adhesive bonding is dry adhesion on the basis of van der Waals forces, similarly to the gecko structures. These structures are detachable and can also be used many times. However, a problem is that the adhesive force of a surface can only be influenced to a limited extent.
Bae et al. (Bae, W. G., Kim, D., Kwak, M. K., Ha, L., Kang, S. M. & Suh, K. Y. (2013a). Enhanced skin adhesive patch with modulus-tunable composite micropillars. Adv. Healthc. Mater., 2, 109-113) were able to show that thin films on end faces of projections can improve the adhesion on soft surfaces such as skin. However, only thin films were used and only inadequately polymerized, which is a problem precisely with use on skin.
The objective of the invention is to provide a structured surface which has adhesive properties and avoids the disadvantages of the prior art. It should be particularly suitable for rough and soft surfaces. Moreover, a method should be provided which enables the production of such structured surfaces.